Remarkably, it seems that Hof can willfully regulate his skin temperature and, potentially, his core body temperature, when exposed to the cold.

The team found that Hof’s unusual breathing leads to an increase in nervous system activity and an uptick in the consumption of glucose in his intercostal muscles, which help form the chest wall. This generates heat that makes its way to the lung tissue, ultimately warming circulating blood there.

“This may counteract a decrease in core body temperature that otherwise would have occurred from cold exposure,” the study surmises.

The brain plays a key role too. Their data suggests that such forced breathing, as well as the state of mind The Iceman enters, seems to active primary control centers involved in modulating sensory pain. Found in the upper brain stem, it’s thought that this control occurs through the release of pain-smothering opioids and cannabinoids.

This study doesn’t stand in isolation. Several others have been conducted, including one in 2014 which, using a group of volunteers mimicking Hof’s breathing, suggested that this technique triggers a flood of adrenaline. This was linked to increased levels of an anti-inflammatory protein that suppressed short-term, painful inflammation and related adverse effects.

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Although it seems like Hof can manipulate his immune system in novel ways, the Iceman himself is known for his hyperbole. Sometimes, as stressed by an excellent rundown of Hof on Discover, he sometimes “begins to step beyond the edges of modern science” by making over-exaggerated and unfounded claims.

The bottom line is that, at present, we need far more peer-reviewed research to be conducted on Hof before we can say with any confidence what’s giving The Iceman his icy resilience.